The invention relates to an optical system permitting the automatic alignment of two motifs comprising alignment marks in the form of optical gratings. The invention more particularly applies to direct photorepetition on silicon.
The manufacture of integrated circuits involves the formation of windows on a substrate making it possible to locate the implantation or treatment. This subtrate is covered with a layer of photosensitive resin. The windows are formed by masking this resin from a mask carried by a reticule. Previously, the proximity of direct contact transfer method was used for this purpose. The processes which are used at present involve transfer by optical projection.
This projection can be carried out in a one-to-one ratio, the mask then being completely projected onto the wafer. This projection can also be carried out by image resolution either by the analysis of the mask by a moving slot or by using photorepetition in a ratio 1:n.
When producing the circuits by direct photorepetition, each motif to be formed is directly projected onto the semiconducting wafer previously covered with a photosensitive resin layer in accordance with a preferred predetermined program, the wafer position being controlled by interferometry in two directions X and Y. Conventionally, photorepetition is carried out by displacing the wafer in two orthogonal directions X and Y.
In all cases, very precise reciprocal positioning of the recticule and the semiconducting material wafer on which the integrated circuit is to be formed is necessary. To this end, numerous alignment processes have been proposed. Generally, supplementary motifs incorporating alignment marks have been used and they are respectively carried by the reticule and the semiconducting wafer. In this process, the number of the marks, their arrangement or their configuration must be such that they permit the alignment in accordance with two reference axes X, Y and the angular alignment of the semiconducting wafer.
One of the most widely used methods consists of superimposing alignment marks on the mask and on the semiconducting wafer by means of transfer optics and observing this superimposition with a microscope having two lenses of the split-field type. The eye of the operator is advantageously replaced by a vidicon tube for display on a television screen.
According to a more elaborate process, the video signal obtained by the television camera can be processed so as to obtain an analog or digital signal which can be used by control circuits. Certain processes use, for example, the scanning of a St. Andrew's cross and its complement to determine the alignment variation which causes a gap between the fronts of the signals.
The apparatus realising these prior art processes have two major disadvantages. The field of the alignment marks carried by the mask and the semiconducting wafer is completely illuminated and onto the images of said marks is superimposed a by no means negligible noise level (parasitic reflection, diffusion, diffraction) leading to a poor signal-to-noise ratio. In addition, the contrast of the silicon marks varies very widely from one integration level to the next, bearing in mind the variations in the oxide thickness or the nature of the deposits (polycrystalline silicon, aluminum). Thus, the alignment quality differs. It is dependent on the contrast on the one hand and on either the keyness of sight of the operator or the resolving power of the associated electronics on the other.
To obviate these disadvantages, it has been proposed to use as the alignment mark, optical networks or fields having a constant or variable spacing, for example in accordance with a pseudo-random code such as codes of the Barker type. Such a process is more particularly described in the European patent application filed in the name of the applicant company and published under No. EP-Al-0015 173 on Sept. 3, 1980 and filed in the United States as application Ser. No. 124,077.
According to the process described in this patent application, a first motif constituted by two optical gratings positioned along two coordinate axes (X, Y) is illuminated by the image of a second motif comprising two optical gratings forming a reference and produces orders of diffraction. According to one of the variants of this application the optical gratings are constituted by a sequence of parallel lines, whose thickness and spatial distribution in a first direction (C) are determined by a pseudo-random code. The lines of the optical gratings of the first motif are also periodically interrupted in a second direction (R) to form an optical grating with a constant spacing. The optical alignment system has optoelectronic means for detecting the intensity of a predetermined part of the orders of diffraction in direction (R).
It is also known that the alignment can be obtained "directly," i.e. through projecting the image of gratings carried by the reticule onto gratings carried by the semiconducting wafer or "sequentially" by using an intermediate alignment motif linked with the photorepeater chassis. Such a process is also described in the aforementioned patent application.
It is also known that for obtaining an automatic alignment it is necessary to create electrical signals which can be used by electronic control circuits associated with the photorepeater. These circuits conventionally act on means for driving or moving the table carrying the semiconducting wafer. Among these signals, it is known to use a bipolar signal in the form of an elongated S passing through zero when alignment is obtained. To obtain this result, it is necessary to modulate the output signals of the optoelectronic detection means.
In the prior art, the means used for obtaining this modulation are generally mechanical. As non-limitative examples, reference is made to the angular modulation of the reflection angle of the light beams used for the alignment by an oscillating mirror, the passage of this beam through an optical plate which also performs an oscillating movement or linearly vibrating about a point of rest of the semiconducting wafer. The latter effect can be obtained by means of piezoelectric motors generally used to equip the X, Y translation table carrying the semiconducting wafer.
These processes have a certain number of disadvantages including the inertia inherent in any mechanical system, the difficulty of obtaining a stable zero (clearly defined point of rest) or in the case of certain of these processes the appearance of optical abberations due to the introduction of optical means onto the path of the light beams used for the alignment and traversed by said beams.